implement-stack-using-queues.js

// Implement the following operations of a stack using queues.
//
//
// 	push(x) -- Push element x onto stack.
// 	pop() -- Removes the element on top of the stack.
// 	top() -- Get the top element.
// 	empty() -- Return whether the stack is empty.
//
//
// Example:
//
//
// MyStack stack = new MyStack();
//
// stack.push(1);
// stack.push(2);  
// stack.top();   // returns 2
// stack.pop();   // returns 2
// stack.empty(); // returns false
//
// Notes:
//
//
// 	You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
// 	Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
// 	You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
//
//


/**
 * @constructor
 */
var Stack = function() {
    this.arr = [];
};

/**
 * @param {number} x
 * @returns {void}
 */
Stack.prototype.push = function(x) {
    this.arr[this.arr.length] = x;
};

/**
 * @returns {void}
 */
Stack.prototype.pop = function() {
    if (this.arr.length === 0) return;
    this.arr.length -= 1;
};

/**
 * @returns {number}
 */
Stack.prototype.top = function() {
    if (this.arr.length === 0) return 0;
    return this.arr[this.arr.length - 1];
};

/**
 * @returns {boolean}
 */
Stack.prototype.empty = function() {
    return this.arr.length === 0;
};